In the field of analog-digital converters (ADC), it is necessary to limit the non-linearity defects of these converters. These defects in fact generate a degradation in the performance of the signal before sampling, the effect of this being to introduce harmonic distortion.
For radar applications in particular, this degradation greatly limits the sensitivity of the radar whose detection threshold must then be raised so as not to generate false detections on the harmonic ranks of the main signal.
To limit this phenomenon, a suitable solution consists in coupling noise with the signal, thus making it possible statistically to limit the periodic passage over the points of non-linearity. However, for this to be effective, the power of this noise must necessarily be fairly significant in order to address the whole set of codes of the converter, but it is also essential that this noise be able to be eliminated after digital acquisition.
Within the framework of a radar application, the useful band of the signal to be processed being known, the choice is to inject noise power into a band not covering the useful band of the signal so as to eliminate the noise easily by high-pass or low-pass filtering as a function of the frequency positioning of the noise.
The generation of this noise and more particularly the implementation of this synthesis can pose a problem of bulkiness, in particular in compact applications. Indeed, for compact applications it is essential to limit the surface area of this functionality whilst the useful bands to be processed are significant, the frequencies involved being very high.
A technical problem to be solved is therefore to carry out the synthesis of analog noise in a high but limited frequency band, for example around 350 MHz to +/−30 MHz, with a relatively high overall power, doing so in a reduced volume.
Solutions for generating analog noise are known. In a first solution, the noise generation is performed by means of an amplified noise diode. This principle necessitates a relatively significant volume and introduces high constraints. Indeed, the noise power is extended over a very wide frequency domain and requires significant filtering in order to preserve only the desired band. Moreover, the noise power is very low after filtering, thus requiring several amplification stages with risks of instability related to the large gain of the chain.
In another solution, the noise is generated on the basis of a digital-analog converter (DAC). The principle is then to synthesize in the digital domain the noise and to convert it into analog via a DAC. With this principle, it is possible to limit the noise band generated but as with any sampled system, it is necessary to filter in analog in order to limit all the image bands. A significant drawback of this solution in regard to the application envisaged is that the frequencies implemented require the use of a DAC with very high sampling frequency, thus necessitating the generation of an additional external clock and extra consumed power, and furthermore not allowing the analog amplification stage to be dispensed with.